Stockholders' Equity - Common stock repurchased (Details) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Shares repurchased related to employee stock-based compensation plans (in shares)	116,151	81,817	3,056,746	2,666,781
Average purchase price per share (in dollars per share)	$ 11.54	$ 10.93	$ 11.34	$ 12.09
Common shares issued (in shares)	397,004	423,112	6,341,950	5,652,886